This invention pertains to bicycle sprockets, in which several sprockets are coaxially mounted spaced apart from each other, and in which a laterally-flexible drive chain means is guided onto one sprocket and caused to shift to another sprocket by laterally shifting a guide sprocket or guide means. Such sprockets have from 12 to about 40 teeth. Shifting the chain from a larger sprocket to a larger sprocket is easily accomplished when the sprockets are nearly the same size, but it becomes more difficult when there is a large difference in sprocket sizes. For example, it is easy to cause a chain to shift from a 14 tooth sprocket to a 16 tooth sprocket, but difficult to cause it to change from a 24 tooth sprocket to a sprocket with 36 teeth; sometimes, under such circumstances, it is difficult to induce the chain to shift at all. What happens is that the chain can't get sufficient engagement on any tooth, and simply rattles from one tooth to the next without changing.
In the past, in order to provide better shifting of the chain, sprockets have been produced in a `skip-tooth` pattern, wherein a tooth is provided for every other chain roller. Sometimes, too, alternate tips of the sprocket teeth are bent in opposite directions to make shifting easier; for the same purpose, sprocket teeth often have sharp, rather than rounded, corners, to make it easier for the sprocket to get a `bite` on the chain and make the desired shift.
Such measures help, but there is still room for improvement.
In order to provide wide ratios in gearing, without employing clusters having large size differences between adjacent sprockets, bicycle manufacturers often supply two or three front sprockets and an adjustable front chain guide, along with a rear cluster having modest size differences in adjacent sprockets. This provides wide overall gear differences without the objectionable large differences in adjacent rear sprocket sizes.
If the chain could be caused to easily make changes between sprockets having large differences in size, the additional front sprockets and the front chain guide mechanism could be often eliminated, with a saving in complexity, weight, and expense.
So, there is need for any device which makes shifting easier between sprockets having large differences in size, and that is what this invention provides.